The present invention relates to cube corner retroreflective sheeting, particularly to cellular retroreflective sheeting for use in the preparation of road markers and signs.
Cube corner retroreflective sheeting and methods for making such sheeting have been described in a number of patents. U.S. Pat. No. 3,684,348 to Rowland describes retroreflective sheeting in which a multiplicity of separately formed, minute cube corner formations having a side edge dimension of less than 0.025 inch are formed by molding the cube corner formations onto a plastic film that serves as the base for the retroreflective sheeting. U.S. Pat. No. 3,810,804 to Rowland describes a method for making such retroreflective sheeting by depositing on a molding surface having cube corner recesses a fluid molding material in an amount to fill the recesses. A pre-formed body member such as a plastic film is applied to the fluid-filled mold and the molding material is hardened and bonded to the body member. The above-mentioned Rowland patents are incorporated herein by this reference.
U.S. Pat. No. 3,712,706 to Stamm, which is incorporated herein by this reference, explains in excellent detail both the theory behind the retroreflective behavior of such cube corner sheeting and methods for forming such sheeting. Briefly, a carefully polished flat blank is ruled with a series of parallel closely spaced grooves using a diamond tool having a point shaped to a precise angle. A second set of parallel grooves is ruled at an angle of 120.degree. to the first set, and a third set of grooves is ruled at an angle of 120.degree. to each of the first two sets to produce minute cube corner prisms having equilateral triangular bases and arranged in an array having hexagonal symmetry. The ruled master is used to form a mold having an array of cube corner recesses. The mold is then used for casting or embossing a series of minute cube corner prisms onto a transparent base sheet. Light entering the base sheet through the side opposite the prisms is reflected within the prisms and directed back through the opposite side of the base sheet toward the source of light.
The intensity of the retroreflective beam from such sheeting is greatest when the incident beam has an angle of incidence of 0.degree.,i.e., is normal to the plane of the reflective sheeting. At higher angles of incidence, e.g., at angles greater than about 30.degree. from the normal, the brightness of the retroreflected beam is a function of the angle about an axis normal to the sheeting, called the azimuthal angle, at which the incident beam strikes the sheeting. When the angle of incidence of a light beam is held constant at a value of, for example, 60.degree. from the normal and the azimuthal angle of the incident beam is varied from 0.degree. to 360.degree., the intensity of the retroreflected beam varies. Rotation of such an array about an axis normal to the array through an angle of 30.degree. produces the maximum difference in orientation of the prisms, whereas rotation through an angle of 60.degree., or any multiple thereof, produces no difference in effective orientation. A graph of the intensity on polar coordinates shows six maxima and six minima at 30.degree. azimuthal intervals.
High retroreflectivity at high angles of incidence is an important characteristic for road signs. When a single large sheet of cube corner retroreflective material is to be used, for example, as the background of a sign, it is a relatively simple matter for the manufacturer of the sheeting to include on the sheet an indication of the proper orientation of the sheet for achieving maximum retroreflectivity at high angles of incidence in a plane roughly parallel to the ground. However, it is customary in sign-making practice to cut up the retroreflective sheeting for making letters and other indicia and for piecing together large backgrounds. Regardless of the orientation of individual pieces of sheeting used to make up the sign, at low angles of incidence the sign will appear uniformly bright. However, at high angles of incidence, some portions of the sign may be oriented to provide a retroreflected beam of higher intensity, and other portions of the sign may be oriented to provide a retroreflected beam of lower intensity. Such a sign would appear to an observer to have an uneven brightness which would at least be unattractive and which could be severe enough to obscure the indicia on the sign.
Thus, great care must be taken to maintain the orientation of the prisms on such retroreflective sheeting when pieces of the sheeting are used to make up signs. Because the individual prisms are so tiny, it can be difficult or impossible to maintain the orientation of the sheeting by visual inspection, and the utility of such sheeting to sign makers is diminished.
The retroreflective efficiency of the sheeting diminishes when the reflecting faces of the prisms become dirty or weathered. Cellular retroreflective sheeting has been described in which a cover sheet is supported in spaced relation to the prisms on a network of narrow intersecting ridges or septa. The cover sheet is sealed to the ridges, thereby providing a plurality of sealed cells, each cell containing a plurality of prisms which are protected within the cell from dirt and weathering. U.S. Pat. No. 4,025,159, which is incorporated herein by this reference, describes cellular retroreflective sheeting and methods for sealing the cover sheet to the network of ridges. For reasons of mechanical strength, simplicity and reflective efficiency, the cells are generally formed as close-packed, regular polygons, notably squares as illustrated in said U.S. Pat. No. 4,025,159. Retroreflective sheeting having such square cells can be oriented visually in either of two perpendicular directions by aligning the sides of the squares. Because the prisms are in an array having hexagonal symmetry, rotation of the entire array by 90 degrees, as would occur in the piecing together of a sign, can result in an effective angular displacement of 30.degree. in the orientation of the prisms on one piece of sheeting with respect to the prisms on another piece of sheeting. Inasmuch as maxima and minima in retroreflective brightness occur at 30.degree. intervals, such misorientation would result in some portions of the sign having maximum retroreflective efficiency in a horizontal plane and other portions of the sign having minimum retroreflective efficiency in the horizontal plane.
It would be desirable, therefore, to have cube corner retroreflective sheeting which could be cut up and pieced together to form a sign without regard for the orientation of the individual pieces.